Heretofore, as a technology for minimizing packet losses, which cannot be prevented by usual layer 3 handover using a technology of a mobile IP, to provide effective means for an internet application requiring a real-time property, a fast handover technology (hereinafter referred to as fast handovers for mobile IP (FMIP)) is disclosed in the following Non-patent Document 1. The FMIP will hereinafter be described with reference to FIGS. 12 to 14.
A wireless communication system shown in FIG. 12 includes an IP network (a communication network) 15 such as an internet, a plurality of subnets (also referred to as subnetworks) 20, 30 connected to the IP network 15, and a mobile terminal (mobile node: MN) 10 connectable to any of these subnets 20, 30. It is to be noted that FIG. 12 shows two subnets 20, 30 as the plurality of subnets 20, 30.
The subnet 20 includes an access router 21 which routes an IP packet (packet data), and a plurality of access points (APs) 22, 23 which form inherent wireless coverages (communicable areas) 24, 25, respectively. These APs 22, 23 are connected to the pAR 21, respectively, and the pAR 21 is connected to the IP network 15. FIG. 12 shows two APs 22, 23 as a plurality of APs 22, 23. The subnet 30 is also constituted of an access router 31 and a plurality of APs 32, 33 in accordance with the same connecting configuration as that of the above subnet 20.
It is to be noted that, here, it is assumed that, in a case where the MN 10 moves into a wireless coverage 34 formed by the AP 32 from the wireless coverage 25 formed by the AP 23 through an overlap area 26, the subnet 20 performs handover to the subnet 30. It is hereinafter assumed that an access router positioned above the AP 23 and connected to the MN 10 before the handover is referred to as the previous access router (pAR) 21 and that an access router positioned above the AP 32 connected to the MN 10 after the handover is referred to as the new access router (nAR) 31.
Moreover, the pAR 21 which is a constituting element of the subnet 20 can communicate with the nAR 31 which is a constituting element of the subnet 30 via the IP network 15. That is, the subnet 20 is connected to the subnet 30 via the IP network 15.
Next, an operation of the FMIP will be described with reference to FIG. 12. The FMIP has two operation modes (a predictive mode and a reactive mode), depending on whether or not the MN 10 receives a fast binding acknowledgement (FBAck) message at a link (the link before the handover) connected to the MN before the handover.
First, the predictive mode of the FMIP in a case where the MN 10 transmits a fast binding update (FBU) message at the link before the handover will be described. FIG. 13 is a sequence chart showing outlines of the operation modes of the FMIP in a case where the MN transmits the FBU message at the link before the handover according to a conventional technology.
For example, when the MN 10 starts moving from the area of the pAR 21 (the wireless coverage 25 of the AP 23) to the area of the nAR 31 (the wireless coverage 34 of the AP 32), Layer 2 detects the movement of the MN. Starting with this detection, Layer 3 starts the handover. This start of the handover is determined by comparing, for example, a reception electric intensity from the AP 23 with that from the AP 32 in the overlap area 26.
In a case where information including AP-ID (identification information of each AP) of the AP 32 as a movement destination is notified from Layer 2, the MN 10 transmits a router solicitation for proxy advertisement (RtSolPr) message including the AP-ID of the AP 32 to the presently connected pAR 21 (step S401). The pAR 21 which has received this RtSolPr message searches for the access router existing in the neighborhood based on the AP-ID of the AP 32 notified from the MN 10 to acquire information of the nAR 31, or acquires the information of the nAR 31 from the information which has already been searched (the information retained by the pAR 21).
Moreover, the pAR 21 transmits, as a response to the RtSolPr message, a proxy router advertisement (PrRtAdv) message including the information of the nAR 31 (e.g., information such as a network prefix of the subnet 30 constituted of the nAR 31) to the MN 10 (step S403). The MN 10 which has received the PrRtAdv message generates new care of address (NCoA) which is an address adaptable to the subnet 30 by use of the network prefix of the subnet 30 included in the PrRtAdv message, a link layer address of the MN 10 itself and the like, and transmits the FBU message including this NCoA to the pAR 21 (step S405).
The pAR 21 which has received the FBU message transmits a handover initiation (HI) message including this NCoA to the nAR 31 in order to confirm whether or not the NCoA generated by the MN 10 is an address usable by the subnet 30 (step S407). On receiving the HI message, the nAR 31 verifies whether or not the NCoA included in this HI message is valid. If the NCoA is valid, a handover acknowledgement (HAck) message in which a status indicating a result of the verification is designated is transmitted to the pAR 21 (step S409). On received the HAck message, the pAR 21 transmits an FBAck message notifying the result to the MN 10 and the nAR 31 (steps S411, S413), and further transmits a packet to be sent to the MN 10 to the nAR 31 (step S415). When the packet to be sent to the MN 10 is transferred from the pAR 21, the nAR 31 buffers the packet. It is to be noted that it is preferable that an operation of transferring the packet to be sent to the MN 10 to the nAR 31 in the step S415 is started immediately after the MN 10 leaves the subnet 20 in step S417 described later.
Moreover, the MN 10 actually starts moving to the subnet 30, and performs, for example, L2 (Layer 2) handover from the AP 23 to the AP 32 (step S417). Immediately after switching the connection to the nAR 31, a fast neighbor advertisement (FNA) message for notifying the connection to the nAR 31 and requiring transmission of the buffered packet is transmitted to the nAR 31 (step S419). The nAR 31 receives this FNA message to transmit the buffered packet to be sent to the MN 10 to the MN 10 (step S421).
Next, the operation mode (the reactive mode) of the FMIP in a case where the MN 10 does not transmit any FBU message at the link before the handover and transmits the FNA (the message including the FBU) at the link after the handover will be described. FIG. 14 is a sequence chart showing an outline of the operation mode of the FMIP in a case where the MN according to the conventional technology transmits the FNA [FBU] message at the link after the handover.
The MN 10 transmits the RtSolPr message (step S501) and receives the PrRtAdv message (step S503) in the same manner as in the operation mode shown in FIG. 13. However, subsequently, the transmission of the FBU message (the step S405 of FIG. 13) in the operation mode shown in FIG. 13 is not performed. The MN actually starts moving to the subnet 30 to perform, for example, L2 handover from the AP 23 to the AP 32 or the like (step S505).
Moreover, immediately after switching the connection to the nAR 31, the MN 10 transmits, to the nAR 31, the FNA message including the FBU message (this message is referred to as the FNA [FBU]) (step S507). The nAR 31 verifies validity of the NCoA included in the FNA message (step S509). In a case where this NCoA is valid, the FBU message is transmitted to the pAR 21 (step S511).
The pAR 21 transmits the FBAck message, as a response to this FBU message, to the nAR 31 (step S513), and further transfers the packet to be sent to the MN 10 to the nAR 31 (step S515). The nAR 31 receives the FBAck message from the pAR 21, and transfers, to the MN 10, the packet to be sent to the MN 10 received from the pAR 21 (step S517). It is to be noted that, to realize the packet transfer of the steps S515 and S517, it is described in Non-patent Document 1 that the pAR 21 may buffer the packet to be sent to the MN 10 received after the MN 10 leaves the subnet 20 to transfer the packet to the nAR 31.
In a case where the FMIP is not used, the packet to be received by the MN 10 is lost from a time when the MN 10 leaves the pAR 21 until the subnet 30 of the nAR 31 completes the binding update. On the other hand, according to the FMIP, when the nAR 31 (in the predictive mode) or the pAR 21 (the reactive mode) buffers the packet to be sent to the MN 10, the MN 10 after the handover can receive all of these packets, and the packet loss during the handover can be minimized. It is to be noted that the above FMIP is one example of a handover mechanism in which the packet to be sent to the MN 10 is buffered during the handover and in which the buffered packet is supplied to the MN 10 after the handover, and there is another handover mechanism capable of achieving a similar object.
Moreover, heretofore, a method of buffering the packet in the router has been considered (e.g., see Non-patent Document 2 described as follows). For example, the router stores the received packet in a queue, and transmits the queued packet. In a case where a predetermined number of or more packets are stored in the queue, an operation of discarding the packet at random or the packet having low priority is performed. It is to be noted that a header of the IPv6 packet is provided with a traffic class and a flow label for a purpose of control of quality of service (QoS). For example, according to these information, the priority, flow and the like of the IPv6 packet can be identified.
Non-Patent Document 1: Rajeev Koodli, “Fast Handovers for Mobile IPv6”, draft-ietf-mipshop-fast-mipv6-03.txt, 25 Oct. 2004
Non-Patent Document 2: Mark Parris, Kevin Jeffay and F. Donelson Smith, “Lightweight Active Router-Queue Management for Multimedia Networking”, ACM/SPIE Multimedia Computing and Networking 1999, January 1999.
According to the FMIP, all the packets for the MN are once buffered in the nAR (the predictive mode) or the pAR (the reactive mode) are once buffered, and then transmitted to the MN which has completed the handover. As a result, the packet loss is largely reduced, but delay of the packet cannot completely be eliminated. On the other hand, among the applications, there is an application which does not permit delay of a predetermined time or more. In a case where a delay time of the packet due to the handover exceeds a delay allowance time inherent in this application, the packet is not required for the MN, and is discarded at the MN. Therefore, according to the FMIP, the packets buffered and supplied to the MN after the handover includes the packet which is discarded at the MN.
When the packet discarded at the MN is buffered and transferred, the following two problems are mainly derived.
1. Useless consumption of resource
A packet retaining area of a buffer is consumed by the unnecessary packet, and a band is consumed by the transfer of the packet.
2. Induction of further packet loss
If the unnecessary packet is transferred, the transfer of the packet which is to reach the MN in a predetermined time delays. As a result, there is a possibility that an extra time (a standby time due to the transfer of the unnecessary packet and the like) is required until the packet reaches the MN and that the packet is discarded at the MN.
It is to be noted that as described above, the pAR and the nAR can perform the QoS control in accordance with the information of the traffic class and the flow label of the header of the packet. However, the information of the traffic class is general information which indicates the priority of the packet, and the packet having the low priority indicated by the information of the traffic class is discarded in accordance with a delay situation and a congestion situation. Therefore, at the pAR and the nAR, the packet having the low priority indicated by the information of the traffic class might be discarded in accordance with a capacity of the buffer for storing the packet. However, even after the elapse of the delay allowance time, a delay sensitive packet having high priority (the delay sensitive packet has a high possibility that the priority is set to be high) is transferred to the MN. Therefore, the above problem of the present invention is not solved.
Moreover, the information of the flow label realizes more specific identification. According to this information of the flow label, for example, the application in which the packet is used and the like can be identified. However, information on the flow of the packet, the delay allowance time of the packet and the application in which the packet is for use can be grasped with an only application level (i.e., only two end nodes including the MN and a correspondent node (CN) which is a communication partner). The pAR and the nAR which simply buffer and transfer the packet in response to the movement of the MN cannot judge the packet to be transferred and the packet which is not required any more and which should be discarded.